In order to optimize the investment in optical fiber links, it is desirable to increase the capacity of said links. This can be achieved by increasing the Spectral Efficiency (SE) of the signals transmitted on said links. A common way to achieve is to use more efficient modulation formats for the transmitted information. This can be used in conjunction with Wavelength Division Multiplexing (WDM). Higher capacity systems with line rate in the range of 40 Gb/s or 100 Gb/s utilize modulation scheme based on Quadrature Phase Shift Keying (QPSK), which codes the information on four phase levels. Therefore, two binary bits can be coded per transmitted symbol. In this manner, the necessary bandwidth of the optical spectrum required to transmit information is used more efficiently, in term of spectral efficiency, enabling the transmission of more information on a fixed bandwidth. For instance, 100 Gb/s signals using Polarization Multiplexed QPSK (PM-QPSK) formats can be transmitted using about 90 channels spaced by 50 GHz on the conventional band (C band) of the spectrum. Such systems are able to transmit roughly 9 Tb/s over a single fiber. Higher capacities can be achieved with more complex modulation formats. For instance, in NPL1 the use of Quadrature Amplitude Modulation format enabled to transmit 101.7 Tb/s. However, this increase in the transmission capacity of the system requires a high complexity in the transmitter and receivers, for a transmission distance limited to 165 km. This is namely not sufficient for long haul applications where transmission over more than 1000 km is required.
In order to increase the capacity of transmission through one fiber while maintaining the possibility of the transmission over long distances, new fibers are being investigated. In NPL2, a Multi Core Fiber (MCF), which consists of several cores conducting optical signals within the same fiber and multicore (MC)—erbium doped fiber amplifier (EDFA), which consists in a fiber amplifier with MCF as gain medium, are used to transmit 40 wavelength of 128 Gb/s PM-QPSK signal over 6160 km of 7-core MCF. This system demonstration highlights the possibility to multiply the system capacity by N, where N is the number of core of MCF, namely N=7 in NPL2, without trading capacity for distance. By using MCF, it is possible to use the multiplicity of cores to spatially multiplex signals, in addition to WDM in each core, increasing the capacity transmitted through fibers without sacrificing the transmitted distance.
Furthermore, as the system capacity can be dramatically increased with the use of MCF, several approaches take advantage of SDM in order to simplify transmitter or receiver used in the transmission system or to reduce their cost. For instance, in NPL3, self homodyne (SH) method is applied on one core of SDM: in that case, the local oscillator (LO) used for coherent reception of signal is generated from the same laser as signal, transmitted through one core and used at the receiver side. This method enables a reduction of the number of used lasers and the simplification of the digital signal processing (DSP) of the receiver. These advantages come at the cost of 1/N of the total system capacity, where N is the number of core of the MCF, which is limited when the number of core grows.
Another approach is presented in NPL4 with the shared carrier reception (SCR) method. In this method, one core is dedicated to the transmission of the non-modulated lightwave carrier, which can be called continuous wave (CW) light, which was used to generate an optical signal. The CW light is received and processed with a dedicated receiver and DSP. According to the result of the DSP of the CW light, which is used in the signal processing, better demodulation is made possible, such as wider compensation of frequency offset between signal and LO as well as compensation of phase noise of the signal. This results in the possibility to use more diverse laser sources and to reduce system cost. The SCR method enables also a reduction of the number of lasers in the system, reducing cost. Again in NPL4, these benefits come at the cost of 1/N of the total system capacity, where N is the number of core of the MCF, which is limited when the number of core grows.